The problem of producing diamond synthetically from carbon, and, more specifically, from graphite has been under attack for a substantial period of time, and, currently, there are two methods which have successfully produced diamond from carbon. One of these is the static method which, although operative, requires the use of complex and expensive equipment to generate the high temperatures and pressures which phase studies have shown are necessary for the conversion of carbon to diamond. A further difficulty with the static method is that the equipment is so complex and massive that the capacity has been extremely limited.
The second method which has been used for the conversion of carbon to diamond depends upon an extremely high-rate shock. The shock-synthesis technique is described in U.S. Pat. No. 3,238,019, which issued on Mar. 1, 1966. According to this patent, a block of carbon is explosively shocked as a result of which extremely high reaction temperatures and pressures are generated, but the yields of diamond were essentially unsatisfactory. It is believed that the low yields obtained using this technique result from reversion of the diamond to carbon, in consequence of the release of shock pressure occurring while the temperature is still high enough so that the diamond is converted to carbon which is the more stable phase when the pressure is released.
Cowan et al. in U.S. Pat. No. 3,401,019 which issued on Sept. 10, 1968, stated that in order to avoid reversion of the diamond produced in shock-synthesis to carbon, it was necessary to cool the reaction product rapidly to below 2000.degree. C, and preferably, to below about 1800.degree. C. For this purpose, they propose to prepare a reaction mixture consisting of carbon with a cooling medium having a high thermal conductivity. Under such circumstances, the rate of cooling of the diamond particles formed would be sufficiently high to prevent excessive graphitization of the diamond particles, the term "graphitization" evidently referring to reversion of diamond particles to graphite. It should be noted that the presence of the adjective "excessive" indicates that some graphitization is unavoidable.
Cowan et al. state that the type of carbon used is not critical but that graphite is generally preferred. A suitable cooling medium is iron, and, in fact, the iron and graphite can be in the form of cast iron. In this form, the carbon particles are mixed in intimate contact with the cooling medium so that heat transfer across the interface between the two phases should be excellent. For this particular combination, diamond is estimated to be produced at a temperature of about 3200.degree. K when carbon having a density of about 1.70 g/cc is shocked to 400 kilobars. Also, diamond is believed to be produced at 2700.degree. K when the carbon is shocked to 300 kilobars.
A critical property for the cooling medium is its shock impedance. Shock impedance is the ratio of the change in applied pressure to the change in material velocity. When the change in pressure is due to an impact which produces a shock wave, the shock impedance is equal to the initial density of the material multiplied by the velocity of the shock wave passing through it, so that the shock impedance is a function of the pressure.
The better cooling media are those having a high shock impedance, say about 3 .times. 10.sup.6 dyne - sec/cm.sup.3 or higher and a high heat capacity, preferably in excess of 0.1 cal/g/.degree. C.
Where the cooling is to take place by heat transfer, the cooling medium should be as free of porosity as possible and generally should have a density of at least 85%, and preferably at least 90% of theoretical.
Phase studies have shown that diamond becomes thermodynamically unstable when the pressure falls to below about 100 kilobars. If the conditions under which dynamic conversion is carried out are such that the pressure stays above this limit for a longer period, then less rapid cooling can be effective. However, when the pressure falls below that at which the diamond is the thermodynamically stable phase, the diamond will revert rapidly to graphite, the decomposition being more rapid the higher the temperature and the lower the pressure. Therefore, it is highly preferable that cooling take place as rapidly as possible so that the temperature may drop below about 2000.degree. C while the pressure is still above 100 kilobars.
Cowan et al. estimate that the cooling time should be less than 0.1 seconds and where the reaction conditions are such that a maximum diamond temperature substantially above 2000.degree. C is reached, even shorter cooling times will be required these being on the order of about 1 millisecond or less. To achieve such rapid rates of cooling, they proprose that the carbon constitute no more than about 85% by volume of the reaction mixture, and where the cooling media are porous, the carbon should constitute no more than 65% by volume of the mixture.
In addition to iron as a suitable cooling medium, other metals are proposed, examples being copper, nickel, aluminum, manganese, etc.
Cowan et al. also propose that the cooling medium have a higher shock impedance than the carbon, such a relationship involving less stringent cooling requirements. Reaction mixtures are prepared by compacting carbon powder and metal shot. The mixture can also be prepared as a casting.
The cooling medium can serve to cool the diamond particles only if the temperature to which the cooling medium is raised by the shock is well below the equilibration temperature of 2000.degree. C or 1800.degree. C. It is for this reason that it is necessary that the shock impedance of the cooling medium be high.
It will be noted that the various metals proposed as cooling media have melting points below the reversion temperature, and well below the reaction temperature for conversion of graphite to diamond. Consequently, the cooling process depends not only on temperature rise of the cooling medium but on fusion as well. It would be desirable that the specific heat as well as the heat of fusion for the various metals be as high as possible. While the specific heats are approximately 0.1 cal/g/.degree. C, heats of fusion are only about 3-5 kcal/mol. Consequently, the change of phase from solid to liquid for the various metals cannot contribute greatly to the cooling effect. A further difficulty is the fact that heat transfer by conduction is a relatively slow process. Since there is some porosity in the system, especially where the graphite and the metal are admixed by compression, there is some heat transfer from the surface of the diamond particles to the exterior of the metal particles by radiation, but, since the metals are highly opaque, heat transfer from the exterior to the interior of metal particles proceeds entirely by conduction.
It is suggested that the yields obtained by shock-synthesis have been limited by the fact that quenching is not sufficiently rapid when quenching depends upon heat transfer by conduction.